The invention relates to a double seat valve with seat cleaning, having two serially arranged closing elements, movable relative to each other, that in the closed position of the double seat valve prevent the overflow of fluids from one valve housing part into another, and that in the closed as well as the open position define a leakage cavity, that is connected to the surroundings of the double seat valve, wherein in the closed position, the first closing element constructed as a slide piston is received in a sealing manner in the connecting opening joining the valve housing parts together, and in the course of its opening movement comes into sealing abutment with a second closing element that is assigned to a second seat and has a second seal, via a center seal acting between the closing elements, and the latter during the further opening movement is led also into an open position, wherein the first closing element has a first seal that seals radially against a cylindrical first seat formed in the connecting opening, with closing elements that can be moved gap-wide independently from each other by a partial lift, in each case, into a seat cleaning position for the purpose of flushing their coaxial seat, wherein the second closing element is moved by a second partial lift in the same direction as the opening movement, and the first closing element is moved by a first partial lift in the direction opposite to the opening movement into its respective seat cleaning position. A double seat valve with seat cleaning of the initially described type that according to Claim 1, permits a cleaning of the respectively uncovered seat through a gap wide opening of its seats is known from the German patent DE 38 35 944 C2. Each of the three seals in the known double seat valve is a discrete seal, wherein each seal performs a single function. The first seal in the first closing element is a purely radial acting seal, which slides into the first seat formed as a cylindrical circumferential area, and seals there are as a consequence of the provided pre-loading (seal with slide engagement). The second seal in the second closing element acts on the second seat, formed cone shaped, so that this can be called radially/axially acting seal (seal with pressure engagement). As a rule, if a seal with pressure engagement is intended, a fixed stop for limiting the end position of the second closing element (closed position) is located at the second seat. The third seal, the so-called center seal, exhibits its effect when during the opening lift, the first closing element comes into abutment with the second closing element via this center seal, and in the course of further opening movement, takes the second closing element along into the completely open position (seal and pressure engagement). Here, the center seal is arranged in a front face facing the leakage cavity, at the second dependently driven closing element.
The known double seat valve, briefly outlined above, has two so-called seat cleaning positions, where the first seat cleaning position is created by the first partial lift of the first closing element in a direction opposite to the opening lift. In the seat cleaning position of the first closing element (this is the lower closing element in reference to the drawing position according to FIG. 3) simultaneously, also a bushing of a drain pipe through the lower valve housing, formed as pressure compensation piston and connected to the first closing element, is uncovered for the purpose of cleaning the assigned seal. The seat cleaning position of the second (upper) closing element is not discussed further in the German patent DE 38 35 944 C2; it occurs through the second partial lift in the same direction as the opening lift. The limitation of the respective quantity of cleaning agent during the seat cleaning occurs through a more or less elaborate, and often insufficient, alignment of the opened gap between the respective seal in the associated seat.
The requirements placed in certain countries on such double seat valves with seat cleaning go beyond the mentioned limitation and are more comprehensive. Thus, for example, in the USA it is required that in the course of the seat cleaning of the other closing element, in the case of larger seal defects or even in the case of the failure of one of the two seat seals of the closing element which is in the closed position, no cleaning agent may pass through via the respective seal defect or via the seat region without seat seal. Under these conditions, not only the requirement to limit the quantity of cleaning agent and avoid a direct impact on the seat region in the course of the seat cleaning, is placed on such a double seat valve, but also the requirement of a possible turbulence free removal of the seat cleaning flow initially into the leakage cavity, and from there into the surroundings, without this seat cleaning flow flowing directly against the respectively closed seat region or impacting it with increasing pressure.
A double seat valve that during the seat cleaning, limits itself solely on the uncovering of the seat in the region of the respectively assigned closing element, and that furthermore realizes a connection of the leakage cavity to the surroundings of the double seat valve, wherein the passage cross-section area of the connection nearly corresponds to the passage cross-section area of the largest nominal pipe size connected to the double seat valve, is described in the company literature “Operating Instructions BAA D620-PMO.32, Double Seat Valve Type D 620 PMO” from Südmo Components GmbH, 73469 Riesbürg, Germany. This double seat valve, due to its ample dimensioning of the connection line between the leakage cavity and the surroundings, is fundamentally suited to prevent a pressure build up in the leakage cavity in the case of larger seal defects. The requirement for avoiding a direct impact on the seat in the course of the seat cleaning, and also of a possibly turbulence free removal of the seat cleaning flow, first into the leakage cavity, and from there into the surroundings, cannot be attained in the known double seat valve. Direct impact is understood to be every speed component of the respective seat cleaning flow, directed orthogonally onto the walls delimiting the seat. It has been shown that any such direct impact leads to a conversion of the kinetic flow energy into static pressure. Depending on the angle of impact of the flow against the wall or body surface, a branching of the flow results in a so-called “branching flow line”, wherein the latter divides the flow into two halves. The branching flow line itself runs up against the so-called “stagnation point”, such that the speed at this location is zero. The pressure increase as a result of this stoppage of the speed is also called “dynamic pressure”. The mechanisms increasing the pressure, described above, generate a leakage flow across the respective gap between the closing element in the assigned seat, and the seat seal that is defective or no longer present at all.
Therefore, a direct impact of the seat cleaning flow on the surface delimiting the leakage cavity is always counterproductive. In the two known double seat valves described above, the first seat cleaning flow which is generated by opening the first closing element by the first partial lift, can impact more or less perpendicularly on the front side periphery of the second closing element, which carries along the center seal. At the point of impact this flow is predominantly redirected to the center of the leakage cavity, and the flow touches, among others, also the front face of the protecting center seal. Furthermore, at the impact point, a branching current line results whose branch facing the seat can generate turbulence there, and a dynamic pressure. The second seat cleaning flow, which is generated by opening a second closing element by a second partial lift, impacts on the front side periphery of the first closing element, and here too, a part of the flow between the branching current line and the seat can be carried to the latter, developing dynamic pressure.
In the patent WO 2007/054 131 A1, measures are already proposed which guarantee drainage, as turbulence free as possible, of the seat cleaning flow into and out of the leakage cavity, and avoid a pressure increasing direct impact on the seat. However, these measures relate to a double seat valve with the features of the German patent DE 196 08 792 A1. The double seat valve described there is distinguished, in particular, in that on the leakage cavity side, each closing element has a known cylindrical appendage on it, which with an associated connection hole between the valve housing parts forms a ring shaped choke gap, and in that a first section of the connecting hole associated with the smaller closing element has a diameter that is smaller than the diameter of a second section of the connecting hole associated with the other closing element, and in that a transition surface is provided between the two sections which forms an obtuse, preferably a perpendicular, deflection angle with the larger diameter section.
It is the object of the present invention to further develop a double seat valve of the initially described type that insures drainage of the seat cleaning flow that is as turbulence free as possible into and out of the leakage cavity, and that reliably avoids a pressure increasing direct impact on the seat.